Publications


Comparison between x-ray scattering and velocity-interferometry measurements from shocked liquid deuterium

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 87 (2013)

K Falk, BJB Crowley, CD Murphy, JS Wark, G Gregori, SP Regan, SX Hu, PB Radha, J Vorberger, DO Gericke, SH Glenzer, AP Jephcoat

The equation of state of light elements is essential to understand the structure of Jovian planets and inertial confinement fusion research. The Omega laser was used to drive a planar shock wave in the cryogenically cooled deuterium, creating warm dense matter conditions. X-ray scattering was used to determine the spectrum near the boundary of the collective and noncollective scattering regimes using a narrow band x-ray source in backscattering geometry. Our scattering spectra are thus sensitive to the individual electron motion as well as the collective plasma behavior and provide a measurement of the electron density, temperature, and ionization state. Our data are consistent with velocity-interferometry measurements previously taken on the same shocked deuterium conditions and presented by K. Falk. This work presents a comparison of the two diagnostic systems and offers a detailed discussion of challenges encountered. ©2013 American Physical Society.


Cosmological growth and feedback from supermassive black holes

ArXiv (2013)

P Mocz, KM Blundell, AC Fabian

We develop a simple evolutionary scenario for the growth of supermassive black holes (BHs), assuming growth due to accretion only, to learn about the evolution of the BH mass function from $z=3$ to 0 and from it calculate the energy budgets of different modes of feedback. We tune the parameters of the model by matching the derived X-ray luminosity function (XLF) with the observed XLF of active galactic nuclei. We then calculate the amount of comoving kinetic and bolometric feedback as a function of redshift, derive a kinetic luminosity function and estimate the amount of kinetic feedback and $PdV$ work done by classical double Fanaroff-Riley II (FR II) radio sources. We also derive the radio luminosity function for FR IIs from our synthesized population and set constraints on jet duty cycles. Around 1/6 of the jet power from FR II sources goes into $PdV$ work done in the expanding lobes during the time the jet is on. Anti hierarchical growth of BHs is seen in our model due to addition of an amount of mass being accreted on to all BHs independent of the BH mass. The contribution to the total kinetic feedback by active galaxies in a low accretion, kinetically efficient mode is found to be the most significant at $z<1.5$. FR II feedback is found to be a significant mode of feedback above redshifts $z\sim 1.5$, which has not been highlighted by previous studies.


Galactic kinematics and dynamics from RAVE stars

ArXiv (2013)

J Binney, B Burnett, G Kordopatis, M Steinmetz, G Gilmore, O Bienayme, J Bland-Hawthorn, B Famaey, EK Grebel, A Helmi, J Navarro, Q Parker, WA Reid, G Seabroke, F Watson, MEK Williams, RFG Wyse, T Zwitter

We analyse the kinematics of ~400000 RAVE stars. We split the sample into hot and cold dwarfs, red-clump and non-clump giants. The kinematics of the clump giants are consistent with being identical with those of non-clump giants. We fit Gaussian velocity ellipsoids to the meridional-plane components of velocity of each star class and give formulae from which the shape and orientation of the velocity ellipsoid can be determined at any location. The data are consistent with the giants and the cool dwarfs sharing the same velocity ellipsoids; sigma_z rises from 21 kms in the plane to sim 55 kms at |z|=2 kpc, while sigma_r rises from 37 kms to 82 kms. At (R,z) the longest axis of one of these velocity ellipsoids is inclined to the Galactic plane by an angle ~0.8 arctan(z/R). We use a novel formula to obtain precise fits to the highly non-Gaussian distributions of v_phi components. We compare the observed velocity distributions with the predictions of a dynamical model fitted to the velocities of stars that lie within ~150 pc of the Sun and star counts towards the Galactic pole. The model accurately reproduces the non-Gaussian nature of the v_r and v_z distributions and provides excellent fits to the data for v_z at all locations. The model v_phi distributions for the cool dwarfs fit the data extremely well, while those for the hot dwarfs have displacements to low v_phi that grow with |z| from very small values near the plane. At |z|>0.5 kpc, the theoretical v_phi distributions for giants show a deficit of stars with large v_phi and the model v_r distributions are too narrow. Systematically over-estimating distances by 20 per cent introduces asymmetry into the model v_r and v_z distributions near the plane and but significantly improves the fits to the data at |z|>0.5 kpc. The quality of the fits lends credence to the assumed, disc-dominated, gravitational potential.


Diffusive shock acceleration at laser-driven shocks: Studying cosmic-ray accelerators in the laboratory

New Journal of Physics 15 (2013)

B Reville, AR Bell, G Gregori

The non-thermal particle spectra responsible for the emission from many astrophysical systems are thought to originate from shocks via a first order Fermi process otherwise known as diffusive shock acceleration. The same mechanism is also widely believed to be responsible for the production of high energy cosmic rays. With the growing interest in collisionless shock physics in laser produced plasmas, the possibility of reproducing and detecting shock acceleration in controlled laboratory experiments should be considered. The various experimental constraints that must be satisfied are reviewed. It is demonstrated that several currently operating laser facilities may fulfil the necessary criteria to confirm the occurrence of diffusive shock acceleration of electrons at laser produced shocks. Successful reproduction of Fermi acceleration in the laboratory could open a range of possibilities, providing insight into the complex plasma processes that occur near astrophysical sources of cosmic rays. © IOP Publishing and Deutsche Physikalische Gesellschaft.


Correlations at large scales and the onset of turbulence in the fast solar WIND

Astrophysical Journal 778 (2013)

RT Wicks, DA Roberts, A Mallet, AA Schekochihin, TS Horbury, CHK Chen

We show that the scaling of structure functions of magnetic and velocity fields in a mostly highly Alfvénic fast solar wind stream depends strongly on the joint distribution of the dimensionless measures of cross helicity and residual energy. Already at very low frequencies, fluctuations that are both more balanced (cross helicity ∼0) and equipartitioned (residual energy ∼0) have steep structure functions reminiscent of "turbulent" scalings usually associated with the inertial range. Fluctuations that are magnetically dominated (residual energy ∼-1), and so have closely anti-aligned Elsasser-field vectors, or are imbalanced (cross helicity ∼1), and so have closely aligned magnetic and velocity vectors, have wide "1/f" ranges typical of fast solar wind. We conclude that the strength of nonlinear interactions of individual fluctuations within a stream, diagnosed by the degree of correlation in direction and magnitude of magnetic and velocity fluctuations, determines the extent of the 1/f region observed, and thus the onset scale for the turbulent cascade. © 2013. The American Astronomical Society. All rights reserved.


Magnetic field generation by Biermann battery and Weibel instability in laboratory shock waves

EAS Publications Series 58 (2012) 23-26

G Gregori, B Reville, F Miniati, RP Drake

Magnetic field generation in the Universe is still an open problem. Possible mechanisms involve the Weibel instability, due to anisotropic phase-space distributions, as well as the Biermann battery, due to misaligned density and temperature gradients. These mechanisms can be reproduced in scaled laboratory experiments. In this contribution we estimate the relative importance of these two processes and explore the laser-energy requirements for producing Weibel dominated shocks. © The Author(s) 2013.


Cosmic-ray acceleration and escape from supernova remnants

Monthly Notices of the Royal Astronomical Society 431 (2013) 415-429

AR Bell, KM Schure, B Reville, G Giacinti

Galactic cosmic-ray (CR) acceleration to the knee in the spectrum at a few PeV is only possible if the magnetic field ahead of a supernova remnant (SNR) shock is strongly amplified by CRs escaping the SNR. A model formulated in terms of the electric charge carried by escaping CRs predicts the maximum CR energy and the energy spectrum of CRs released into the surrounding medium. We find that historical SNRs such as Cas A, Tycho and Kepler may be expanding too slowly to accelerate CRs to the knee at the present time. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


High-power laser experiments to study collisionless shock generation

EPJ Web of Conferences 59 (2013)

Y Sakawa, Y Kuramitsu, T Morita, T Norimatsu, H Takabe, T Kato, H Tanji, T Ide, T Tsubouchi, K Nishio, M Kuwada, H Ide, C Gregory, N Woolsey, K Schaar, C Murphy, G Gregori, A Diziere, A Pelka, M Koenig, S Wang, Q Dong, Y Li, H-S Park, S Ross, N Kugland, D Ryutov, B Remington, A Spitkovsky, D Froula

A collisionless Weibel-instability mediated shock in a self-generated magnetic field is studied using two-dimensional particle-in-cell simulation [Kato and Takabe, Astophys. J. Lett. 681, L93 (2008)]. It is predicted that the generation of the Weibel shock requires to use NIF-class high-power laser system. Collisionless electrostatic shocks are produced in counter-streaming plasmas using Gekko XII laser system [Kuramitsu et al., Phys. Rev. Lett. 106, 175002 (2011)]. A NIF facility time proposal is approved to study the formation of the collisionless Weibel shock. OMEGA and OMEGA EP experiments have been started to study the plasma conditions of counter-streaming plasmas required for the NIF experiment using Thomson scattering and to develop proton radiography diagnostics. © Owned by the authors, published by EDP Sciences, 2013.


Laboratory experiments on plasma jets in a magnetic field using high-power lasers

EPJ Web of Conferences 59 (2013)

K Nishio, M Kuwada, Y Sakawa, Y Kuramitsu, T Morita, M Koga, T Kato, T Norimatsu, H Takabe, T Ide, C Gregory, N Woolsey, C Murphy, G Gregori, K Schaar, A Diziere, M Koenig, A Pelka, S Wang, Q Dong, Y Li

The experiments to simulate astrophysical jet generation are performed using Gekko XII (GXII) HIPER laser system at the Institute of Laser Engineering. In the experiments a fast plasma flow generated by shooting a CH plane (10 μm thickness) is observed at the rear side of the plane. By separating the focal spot of the main beams, a non-uniform plasma is generated. The non-uniform plasma flow in an external magnetic field (0.2∼0.3 T) perpendicular to the plasma is more collimated than that without the external magnetic field. The plasma β, the ratio between the plasma and magnetic pressure, is ≠1, and the magnetic Reynolds number is ∼150 in the collimated plasma. It is considered that the magnetic field is distorted by the plasma flow and enhances the jet collimation. © Owned by the authors, published by EDP Sciences, 2013.


Alignment and scaling of large-scale fluctuations in the solar wind

ArXiv (2012)

RT Wicks, A Mallet, TS Horbury, CHK Chen, AA Schekochihin, JJ Mitchell

We investigate the dependence of solar wind fluctuations measured by the Wind spacecraft on scale and on the degree of alignment between oppositely directed Elsasser fields. This alignment controls the strength of the non-linear interactions and, therefore, the turbulence. We find that at scales larger than the outer scale of the turbulence the Elsasser fluctuations become on average more anti-aligned as the outer scale is approached from above. Conditioning structure functions using the alignment angle reveals turbulent scaling of unaligned fluctuations at scales previously believed to lie outside the turbulent cascade in the `1/f range'. We argue that the 1/f range contains a mixture of non-interacting anti-aligned population of Alfv\'{e}n waves and magnetic force-free structures plus a subdominant population of unaligned cascading turbulent fluctuations.


Overview of physics results from MAST towards ITER/DEMO and the MAST Upgrade

Nuclear Fusion 53 (2013)

H Meyer, RJ Akers, SY Allan, LC Appel, N Ben Ayed, CD Challis, IT Chapman, D Ciric, G Colyer, NJ Conway, M Cox, BJ Crowley, SC Cowley, G Cunningham, A Darke, RO Dendy, D Dickinson, MD Driscoll, M Dunstan, S Elmore, AR Field, G Fishpool, S Freethy, W Fundamenski, L Garzotti, YC Ghim, MP Gryaznevich, J Harrison, E Havlíčková, NC Hawkes, TC Hender, DF Howell, D Keeling, A Kirk, YQ Liu, B Lloyd, GP Maddison, J Mailloux, R Martin, GJ McArdle, KG McClements, C Michael, F Militello, AW Morris, DG Muir, G Naylor, MR O'Brien, A Patel, SD Pinches, MN Price, CM Roach, M Romanelli, S Saarelma, R Scannell, SE Sharapov, V Shevchenko, S Shibaev, D Stork, J Storrs, A Sykes, D Taylor, N Thomas-Davies, A Thornton, MR Turnyanskiy, M Valovič, G Voss, SEV Warder, IG Abel, M Barnes, E Highcock, P Hill, FI Parra, AA Schekochihin, A Allan, NC Barratt, P Denner, B Dudson, KJ Gibson, K Imada, T O'Gorman, GJ Tallents, RGL Vann, HR Wilson, O Asunta, JW Bradley, J Canik, J Seidl, P Cahyna, J Horáček, S Sangaroon, I Wodniak, M Cecconello, A Danilov, AY Dnestrovsky, Y Dnestrovsky, MFM De Bock, G De Temmerman, T Morgan, D Dunai, S Zoletnik, P Dura, D Higgins, B Hnat, RJ Lake, B McMillan, JR Robinson, E Verwichte, WW Heidbrink, MJ Hole, O Jones, E Kaveeva, P Molchanov, V Rozhansky, P Voskoboynikov, M Kočan, M Lehnen, Y Liang, HJ Leggate, MK Lilley, D Temple, SW Lisgo, S Mordijck, E Nardon, P Tamain, V Naulin, AH Nielsen, S Pamela, A Saveliev, R Zagôrski

New diagnostic, modelling and plant capability on the Mega Ampère Spherical Tokamak (MAST) have delivered important results in key areas for ITER/DEMO and the upcoming MAST Upgrade, a step towards future ST devices on the path to fusion currently under procurement. Micro-stability analysis of the pedestal highlights the potential roles of micro-tearing modes and kinetic ballooning modes for the pedestal formation. Mitigation of edge localized modes (ELM) using resonant magnetic perturbation has been demonstrated for toroidal mode numbers n = 3, 4, 6 with an ELM frequency increase by up to a factor of 9, compatible with pellet fuelling. The peak heat flux of mitigated and natural ELMs follows the same linear trend with ELM energy loss and the first ELM-resolved T measurements in the divertor region are shown. Measurements of flow shear and turbulence dynamics during L-H transitions show filaments erupting from the plasma edge whilst the full flow shear is still present. Off-axis neutral beam injection helps to strongly reduce the redistribution of fast-ions due to fishbone modes when compared to on-axis injection. Low-k ion-scale turbulence has been measured in L-mode and compared to global gyro-kinetic simulations. A statistical analysis of principal turbulence time scales shows them to be of comparable magnitude and reasonably correlated with turbulence decorrelation time. T inside the island of a neoclassical tearing mode allow the analysis of the island evolution without assuming specific models for the heat flux. Other results include the discrepancy of the current profile evolution during the current ramp-up with solutions of the poloidal field diffusion equation, studies of the anomalous Doppler resonance compressional Alfvén eigenmodes, disruption mitigation studies and modelling of the new divertor design for MAST Upgrade. The novel 3D electron Bernstein synthetic imaging shows promising first data sensitive to the edge current profile and flows. © 2013 IAEA, Vienna.


Gamma-ray emission in near critical density plasmas

Plasma Physics and Controlled Fusion 55 (2013)

CS Brady, TD Arber, CP Ridgers, AR Bell

Previous work on the interaction of high power lasers with high density targets have identified that emission is primarily from interaction within the skin layer at the target front (e.g. Ridgers et al 2012 Phys. Rev. Lett. 108 165006). This mechanism is inefficient when compared to Reinjected Electron Synchrotron Emission (RESE) for laser interaction with low density solids (Brady et al 2012 Phys. Rev. Lett. 109 245006). However, these detailed analyses of the emission mechanisms were mainly based on 1D simulations and so did not incorporate some important 2D effects. In this paper these 1D emission mechanisms are confirmed to still exist in 2D with comparable properties and a new, intrinsically 2D, emission mechanism, termed edgeglow, is described which can convert 4-5% of the laser energy into gamma-ray energy. © 2013 IOP Publishing Ltd.


Stream-orbit misalignment I: The dangers of orbit-fitting

ArXiv (2013)

JL Sanders, J Binney

Tidal streams don't, in general, delineate orbits. A stream-orbit misalignment is expected to lead to biases when using orbit-fitting to constrain models for the Galactic potential. In this first of two papers we discuss the expected magnitude of the misalignment and the resulting dangers of using orbit-fitting algorithms to constrain the potential. We summarize data for known streams which should prove useful for constraining the Galactic potential, and compute their actions in a realistic Galactic potential. We go on to discuss the formation of tidal streams in angle-action space, and explain why, in general, streams do not delineate orbits. The magnitude of the stream-orbit misalignment is quantified for a logarithmic potential and a multi-component Galactic potential. Specifically, we focus on the expected misalignment for the known streams. By introducing a two-parameter family of realistic Galactic potentials we demonstrate that assuming these streams delineate orbits can lead to order one errors in the halo flattening and halo-to-disc force ratio at the Sun. We present a discussion of the dependence of these results on the progenitor mass, and demonstrate that the misalignment is mass-independent for the range of masses of observed streams. Hence, orbit-fitting does not yield better constraints on the potential if one uses narrower, lower-mass streams.


Cosmic ray acceleration

Astroparticle Physics 43 (2013) 56-

AR Bell


Dense electron-positron plasmas and bursts of gamma-rays from laser-generated quantum electrodynamic plasmas

Physics of Plasmas 20 (2013)

CP Ridgers, AR Bell, CS Brady, K Bennett, TD Arber, R Duclous, JG Kirk

In simulations of a 12.5 PW laser (focussed intensity I = 4 × 10 23 Wcm - 2) striking a solid aluminum target, 10% of the laser energy is converted to gamma-rays. A dense electron-positron plasma is generated with a maximum density of 10 26 m - 3, seven orders of magnitude denser than pure e e plasmas generated with 1PW lasers. When the laser power is increased to 320 PW (I = 10 25 Wcm - 2), 40% of the laser energy is converted to gamma-ray photons and 10% to electron-positron pairs. In both cases, there is strong feedback between the QED emission processes and the plasma physics, the defining feature of the new "QED-plasma" regime reached in these interactions. © 2013 AIP Publishing LLC.


Visualizing electromagnetic fields in laser-produced counter-streaming plasma experiments for collisionless shock laboratory astrophysics

Physics of Plasmas 20 (2013)

NL Kugland, JS Ross, SH Glenzer, C Huntington, MC Levy, D Martinez, C Plechaty, BA Remington, DD Ryutov, H-S Park, P-Y Chang, G Fiksel, DH Froula, RP Drake, M Grosskopf, C Kuranz, G Gregori, J Meinecke, B Reville, M Koenig, A Pelka, A Ravasio, Y Kuramitsu, T Morita, Y Sakawa, H Takabe, E Liang, F Miniati, R Presura, A Spitkovsky

Collisionless shocks are often observed in fast-moving astrophysical plasmas, formed by non-classical viscosity that is believed to originate from collective electromagnetic fields driven by kinetic plasma instabilities. However, the development of small-scale plasma processes into large-scale structures, such as a collisionless shock, is not well understood. It is also unknown to what extent collisionless shocks contain macroscopic fields with a long coherence length. For these reasons, it is valuable to explore collisionless shock formation, including the growth and self-organization of fields, in laboratory plasmas. The experimental results presented here show at a glance with proton imaging how macroscopic fields can emerge from a system of supersonic counter-streaming plasmas produced at the OMEGA EP laser. Interpretation of these results, plans for additional measurements, and the difficulty of achieving truly collisionless conditions are discussed. Future experiments at the National Ignition Facility are expected to create fully formed collisionless shocks in plasmas with no pre-imposed magnetic field. © 2013 AIP Publishing LLC.


Surface waves and electron acceleration from high-power, kilojoule-class laser interactions with underdense plasma

New Journal of Physics 15 (2013)

L Willingale, AGR Thomas, A Maksimchuk, C Zulick, K Krushelnick, PM Nilson, RS Craxton, TC Sangster, C Stoeckl, H Chen, J Cobble, PA Norreys, RHH Scott

Experiments were performed on the Omega EP laser facility to study laser pulse propagation, channeling phenomena and electron acceleration from high-intensity, high-power laser interactions with underdense plasma. A CH plasma plume was used as the underdense target and the interaction of the laser pulse channeling through the plasma was imaged using proton radiography. High-energy electron spectra were measured for different experimental laser parameters. Structures observed along the channel walls are interpreted as having developed from surface waves, which are likely to serve as an injection mechanism of electrons into the cavitated channel for acceleration via direct laser acceleration mechanisms. Two-dimensional particle-in-cell simulations give good agreement with these channeling and electron acceleration phenomena. © IOP Publishing and Deutsche Physikalische Gesellschaft.


Pair plasma cushions in the hole-boring scenario

Plasma Physics and Controlled Fusion 55 (2013)

JG Kirk, AR Bell, CP Ridgers

Pulses from a 10 PW laser are predicted to produce large numbers of gamma-rays and electron-positron pairs on hitting a solid target. However, a pair plasma, if it accumulates in front of the target, may partially shield it from the pulse. Using stationary, one-dimensional solutions of the two-fluid (electron-positron) and Maxwell equations, including a classical radiation reaction term, we examine this effect in the hole-boring scenario. We find the collective effects of a pair plasma 'cushion' substantially reduce the reflectivity, converting the absorbed flux into high-energy gamma-rays. There is also a modest increase in the laser intensity needed to achieve threshold for a non-linear pair cascade. © 2013 IOP Publishing Ltd.


Numerical modeling of the sensitivity of x-ray driven implosions to low-mode flux asymmetries

Physical Review Letters 110 (2013)

RHH Scott, PA Norreys, DS Clark, DK Bradley, DA Callahan, MJ Edwards, SW Haan, OS Jones, BK Spears, MM Marinak, RPJ Town, LJ Suter

The sensitivity of inertial confinement fusion implosions, of the type performed on the National Ignition Facility (NIF), to low-mode flux asymmetries is investigated numerically. It is shown that large-amplitude, low-order mode shapes (Legendre polynomial P), resulting from low-order flux asymmetries, cause spatial variations in capsule and fuel momentum that prevent the deuterium and tritium (DT) "ice" layer from being decelerated uniformly by the hot spot pressure. This reduces the transfer of implosion kinetic energy to internal energy of the central hot spot, thus reducing the neutron yield. Furthermore, synthetic gated x-ray images of the hot spot self-emission indicate that P shapes may be unquantifiable for DT layered capsules. Instead the positive P asymmetry "aliases" itself as an oblate P in the x-ray images. Correction of this apparent P distortion can further distort the implosion while creating a round x-ray image. Long wavelength asymmetries may be playing a significant role in the observed yield reduction of NIF DT implosions relative to detailed postshot two-dimensional simulations. © 2013 American Physical Society.


Blowing cold flows away: the impact of early AGN activity on the formation of a brightest cluster galaxy progenitor

ArXiv (2012)

Y Dubois, C Pichon, J Devriendt, J Silk, M Haehnelt, T Kimm, A Slyz

Supermassive black holes (BH) are powerful sources of energy that are already in place at very early epochs of the Universe (by z=6). Using hydrodynamical simulations of the formation of a massive M_vir=5 10^11 M_sun halo by z=6 (the most massive progenitor of a cluster of M_vir=2 10^15 M_sun at z=0), we evaluate the impact of Active Galactic Nuclei (AGN) on galaxy mass content, BH self-regulation, and gas distribution inside this massive halo. We find that SN feedback has a marginal influence on the stellar structure, and no influence on the mass distribution on large scales. In contrast, AGN feedback alone is able to significantly alter the stellar-bulge mass content by quenching star formation when the BH is self-regulating, and by depleting the cold gas reservoir in the centre of the galaxy. The growth of the BH proceeds first by a rapid Eddington-limited period fed by direct cold filamentary infall. When the energy delivered by the AGN is sufficiently large to unbind the cold gas of the bulge, the accretion of gas onto the BH is maintained both by smooth gas inflow and clump migration through the galactic disc triggered by merger-induced torques. The feedback from the AGN has also a severe consequence on the baryon mass content within the halo, producing large-scale hot superwinds, able to blow away some of the cold filamentary material from the centre and reduce the baryon fraction by more than 30 per cent within the halo's virial radius. Thus in the very young universe, AGN feedback is likely to be a key process, shaping the properties of the most massive galaxies.